IT’S been 27 years since the United States hosted a world’s fair – known to the rest of the world as an expo. So we agree with the Bay Area Council, the regional business group based in San Francisco, which thinks it is about time.

When? 2020. Where? Moffett Field, northwest of San Jose. Why? To rally Silicon Valley to reclaim its image as the center of the tech universe and, organizers hope, to pump billions of dollars into the California economy.

It’s an exciting thought.

Given the success of last year’s Shanghai Expo, it’s well worth the time to analyze costs and determine how reliable the projected return on investment might be.

But one thing has to happen immediately: The United States has to rejoin the Bureau of International Expositions, which chooses expo locations much like the International Olympics Committee chooses game sites. Membership only costs about $33,000 a year.

Bay Area Council CEO Jim Wunderman, of the group proposing to bring back the world’s fair to the Golden State, says if necessary, he’ll pass the hat. We think businesses from all over the state should back it – it’s good for down here, too.

The U.S. dropped out of the Bureau of International Expositions during a fit of isolationism a few years back. Today, competing in a global marketplace, we need to be in this game no matter where the expos are.

We almost didn’t have a pavilion in Shanghai, but at the last minute Secretary of State Hillary Clinton twisted some corporate arms to pay for one. Americans say it was a tad too commercial, but Chinese visitors kept it packed. With more time, it can be done better.

Last fall the Bay Area Council and then-Gov. Arnold Schwarzenegger went to Shanghai to launch a bid for the 2020 Expo.

Wunderman thinks it’s ours to lose if the U.S. pays those dues.

A study commissioned by the council predicts that the event would spark $5.6 billion in new economic activity in Northern California and create 42,000 jobs, about the number the Silicon Valley lost in the past few years.

We’re always leery of studies funded by advocates of a project, but it’s common sense that the ripple effects of a well-done event would be significant.

The Shanghai Expo drew some 73 million visitors over six months. The council predicts 25 million here, closer to average. Shanghai was a more exotic destination than most. China also had the advantage (for big-event purposes) of being a dictatorship: It could spend gazillions on an expo without having to worry about Republicans and Democrats bickering.

This presents a challenge for the California bid. Given the state of the economy today and projections well into this decade, the plan will have to rely on private investment. The only realistic hope of public dollars would be to expedite projects like road or transit improvements already in the pipeline.

None of this will even be possible unless the U. S. agrees to sign that $33,000 dues check. Political leaders from not just the Bay Area but all of California – and from other U.S. tech hubs (Pasadena, Monrovia, Los Angeles, Irvine, etc.), for that matter – should be pressing the case.